port 2P step 1: vendor CPython 2.7.18 as the mtpython static library

The embedded 40250 script layer needs its own interpreter on five platforms.
2.7 is end-of-life, so nothing can be fetched from the target SDKs and the
source is vendored (official 2.7.18 tarball, trimmed to 25MB; Lib/ stays for
step 3's python27.zip).

cpython-2.7.18/CMakeLists.txt builds one `mtpython` static library from exactly
the 133 objects the reference libpython2.7.a contains, off by default behind
-DMTGODOT_EMBED_PYTHON=ON. The source list and the built-in module table
(config/config.c, 39 entries) are shared; pyconfig.h is a probe result and is
not, so each platform keeps its own under config/<platform>/, regenerated by
tools/py_embed/gen_pyconfig.sh and checked against the shared table.

Three vendor patches, documented in docs/THIRD-PARTY.md: configure/configure.ac
learn arm64 on macOS, and posixmodule.c undefines the process-control calls it
hard-defines past pyconfig.h when building for iOS.

macOS (ctest python.embed: every builtin imports, codecs and pickle work off the
vendored Lib/), Android arm64 and iOS arm64 build. Linux needs a Linux host and
Windows needs a MinGW-vs-MSVC decision; both are noted for step 4.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/third_party/cpython-2.7.18/PC/make_versioninfo.c

@@ -0,0 +1,38 @@
+#include <stdio.h>
+#include "patchlevel.h"
+/*
+ * This program prints out an include file containing fields required to build
+ * the version info resource of pythonxx.dll because the resource compiler
+ * cannot do the arithmetic.
+ */
+/*
+ * FIELD3 is the third field of the version number.
+ * This is what we'd like FIELD3 to be:
+ *
+ * #define FIELD3 (PY_MICRO_VERSION*1000 + PY_RELEASE_LEVEL*10 + PY_RELEASE_SERIAL)
+ *
+ * but that neither gives an error nor comes anywhere close to working.
+ *
+ * For 2.4a0,
+ * PY_MICRO_VERSION = 0
+ * PY_RELEASE_LEVEL = 'alpha' = 0xa
+ * PY_RELEASE_SERIAL = 0
+ *
+ * gives FIELD3 = 0*1000 + 10*10 + 0 = 100
+ */
+int main(int argc, char **argv)
+{
+    printf("/* This file created by make_versioninfo.exe */\n");
+    printf("#define FIELD3 %d\n",
+        PY_MICRO_VERSION*1000 + PY_RELEASE_LEVEL*10 + PY_RELEASE_SERIAL);
+    printf("#define MS_DLL_ID \"%d.%d\"\n",
+           PY_MAJOR_VERSION, PY_MINOR_VERSION);
+    printf("#ifndef _DEBUG\n");
+    printf("#define PYTHON_DLL_NAME \"python%d%d.dll\"\n",
+           PY_MAJOR_VERSION, PY_MINOR_VERSION);
+    printf("#else\n");
+    printf("#define PYTHON_DLL_NAME \"python%d%d_d.dll\"\n",
+           PY_MAJOR_VERSION, PY_MINOR_VERSION);
+    printf("#endif\n");
+    return 0;
+}